The quantification or assay of chemical and/or biochemical constituents within biological fluids, such as blood, urine, and saliva, and within biological fluid fractions or derivatives such as blood serum and blood plasma, is of ever increasing importance for medical diagnosis and treatment, as well as the quantification of exposure to therapeutic drugs, intoxicants, hazardous chemicals, and the like. One such common application is the measurement of blood glucose levels in diabetics.
Widely accepted assays involve measuring a change in a physical characteristic of the fluid being tested or an element of such fluid when exposed to a particular energy source. These physical characteristics are typically an electrical, magnetic, fluidic, or optical property of the fluid or a component thereof. For example, with a colorimetric assay system, an optical property may be monitored wherein a change in light absorption of the fluid can be related to an analyte concentration in, or a property of, the fluid.
To carry out the assays, a disposable test strip, pad, or the like, is employed in conjunction with a meter. A sample of the biological fluid to be tested is provided. When the biological fluid is blood, a sample is typically acquired by means of a finger stick. The fluid sample is then deposited in a designated measurement area of the test strip, which contains reagents selected for the particular assay being conducted. The test strip, or at least a portion thereof, is placed in a receptacle area or test strip holder within the meter. The meter is capable of receiving a signal originating in a measurement area of the test strip and determining the existence and/or concentration of the constituent or analyte of interest. Examples of assay systems that employ these types of disposable test strips and meters may be found in U.S. application Ser. No. 09/333,765, filed Jun. 15, 1999, and Ser. No. 09/356,248, filed Jul. 16, 1999; and in U.S. Pat. Nos. 4,935,346, 5,049,487, 5,304,468 and 5,563,042, the disclosures of which are herein incorporated by reference.
Often, the measurement area of the test strip is defined by a small aperture within the surface of the test strip. Placed over and covering the aperture on one side of the test strip is a hydrophilic material, e.g., a membrane, matrix, layer, or the like, containing reagent(s) suitable for determining the existence and/or the concentration of the particular analyte of interest. The sampled fluid is deposited on the opposite side of the test strip within the aperture whereby the fluid is then absorbed into the hydrophilic matrix. Such a test strip configuration is used, for example, in calorimetric measurement systems; see, e.g., U.S. Pat. No. 5,563,042. Such systems employ meters, such as a diffuse reflectance spectrophotometer with accompanying software, which can be made to automatically transmit a light source at a particular wavelength and then read reflectance, of the test sample at certain points in time, and, using calibration factors, determine the concentration of analyte in the sampled fluid.
In order to obtain an accurate measurement of the fluid sample deposited within the aperture, it is necessary to properly position the test strip within the test strip holder and align the aperture of the test strip with the light source, typically a high-intensity light emitting diode (LED), within the meter. Improper positioning of the test strip can result, for example, from a slight rebound of the test strip as its distal or insertion end is caused to contact the edge of the strip holder. Also, some shifting or slipping of the test strip may occur after it has been placed within the meter.
To facilitate proper positioning and alignment of the test strip within the test strip holder, a notch or a cut-out is formed within an edge of a test strip which is to be aligned with a corresponding or mating alignment pin within the inner edge of the test strip holder. This has not been completely successful as the strip is still able, to some degree, to shift from side-to-side when the strip is not fully inserted. Such movement or “play” in the position of the test strip increases the likelihood that the test strip will be improperly or not completely inserted or misaligned within the meter. As a result of this misalignment, the measurement aperture of the test strip may not be centered with respect to the light source, which may then result in an incorrect measurement.
Often, to compensate for this likelihood of misalignment and the resulting incorrect measurement, a larger aperture requiring a greater volume of the biological fluid, e.g., blood, being tested is used so as to provide a larger measurement area within the test strip. A disadvantage of using a greater volume of sampled fluid, blood in particular, to saturate this area of exposed hydrophilic matrix, is the need to draw a greater volume of blood sample from the patient. This requisite greater volume of sampled fluid requires use of a blood sample size which is rather large for a typical finger stick, thus necessitating use of a larger diameter needle and/or deeper penetration into the skin. These factors can increase the discomfort and pain felt by the patient, and may be difficult to achieve for those individuals whose capillary blood does not readily express. As this sampling process may be repeated frequently within a single day, for many diabetics, an increase in pain quickly becomes less tolerable or intolerable all together.
As such, there is a continuing need for a test device for use in analyte concentration measurement that is easy to insert into and self-aligning within a meter, highly resistant to rebounding upon insertion and to movement once operatively placed within the meter, and minimizes the volume of the sample of biological fluid that is necessary to ensure an accurate measurement.
RELEVANT LITERATURE 
Patents and publications of interest include: U.S. Pat. Nos. 4,935,346, 5,049,487, 5,304,468 and 5,563,042.